1. Field of the Invention
The present invention relates to methods for locating porous, permeable soils having high percolation rates, such as sands or gravels, at locations having both permeable and impermeable soils present. Particularly, the present invention relates to methods for locating porous, permeable soils by measuring the temperature of the earth's surface at a number of points within the bounds of a selected location, then, comparing the measured temperatures to identify those points having earth surface temperatures higher than those at other points within the location. Soils beneath points having higher temperatures tend to be more porous and permeable and to have higher percolation rates than soils beneath points having lower temperatures.
The ability to identify and locate porous and permeable soils is useful in environmental matters which involve land drainage patterns. Porous, permeable soils with higher percolation rates drain water faster than more impermeable soils. One particular use for the method of the present invention is to locate surface deposits of porous, permeable soils which are suitable for use as evaporation fields for septic tank systems.
2. Description of Pertinent Art
In U.S. Pat. No. 4,476,716, I disclosed methods for predicting the likelihood that oil or gas deposits are present below selected locations. These methods were based upon the discovery that earth temperatures tend to be lower at locations where oil or gas deposits exist than earth temperatures at the same depth at locations where oil or gas deposits do not exist.
In U.S. patent application Ser. No. 07/923,477, filed Aug. 3, 1992 and copending with this application, I disclosed methods for predicting whether oil or gas deposits are likely to be present beneath selected locations, which methods employ differences in the temperature of the earths surface from point to point. These methods for locating oil or gas deposits are based upon the discovery that earth surface temperatures above oil or gas deposits are slightly, but measurably, less than the average earth surface temperature for the surrounding area.
In developing these methods for finding oil or gas deposits, I observed that the earth's surface and near surface temperatures are affected by a wide variety of environmental factors, including: seasonal temperature fluctuations; diurnal effects; fluctuations in incident radiation; ambient atmospheric temperature; atmospheric humidity; wind velocity; cloud cover; atmospheric clarity; precipitation; reflectance and emmissivity of the earth's surface; vegetative cover; surface elevation above sea level; local topography; surface features; and, pertinent to the present invention, soil moisture and soil type. The effect of all these factors taken together tends to obscure the effect of any one factor, and my inventions have involved methods for extracting meaningful information from earth temperature measurement data.
The present invention arose from observing workers at homesites on Washington Island, Wis. The workers were attempting to locate soils having sufficient porosity and permeability to serve as evaporation fields for septic tank systems. Washington Island is comprised of thin soils overlying Silurian limestone. Many of the soils are relatively impermeable silts and clays in which water tends to pool. Interspersed in these impermeable soils are sand and gravel beds, such as remnants of ancient beaches, which are porous and permeable, have high water percolation rates and are suitable for septic tank evaporation field sites. At each homesite, the workers were digging, with tractor mounted back-hoes, to locate sand and gravel beds suitable for septic tank evaporation fields. The digging process was slow, destroyed vegetation, and created unnecessary holes at each homesite. From these observations of the workmen's efforts, it became apparent that an improved method for locating the porous, permeable sand and gravel beds was desirable.
Septic tank systems treat sewage, generally from a single family or other small source. Such septic tank systems comprise:
a septic tank where sewage is received for digestion by anaerobic bacteria and for production of waste water; PA1 a drain line for transporting waste water from the septic tank; field lines for receiving waste water from the drain line; and PA1 an evaporation field comprised of porous, permeable material, such as sand or gravel, where waste water received from the field lines is evaporated. PA1 selecting a location likely to have both porous, permeable soils and impermeable soils present; PA1 selecting a time period when ambient temperatures and soil surface temperatures are above the mean annual atmospheric temperature for the location, and when the soil is neither frozen nor water logged; PA1 measuring, during the selected time period, earth surface temperatures at a plurality of points within the bounds of the location; selecting points with earth surface temperatures higher than earth surface temperatures at other points within the location, as being located in areas with higher permeability then areas around points with lower temperatures.
Waste water from a septic tank is evaporated in the evaporation field, and should never be allowed to drain away, either into surface waters or into subsurface aquifers. Consequently, surface deposits of porous, permeable materials having high percolation rates are selected as sites for septic tank evaporation fields. Preferably, the surface deposits are underlain by more impermeable deposits which prevent water from percolating downward into subsurface aquifers.
Sands and gravels have the desired porosity, permeability and high water percolation rates which make such deposits suitable for septic tank evaporation fields. Air, as well as water, circulates through the porous and permeable sand and gravel beds, increasing the rate of evaporation. Heavier soils, such as silt and clay, are fine grained and have little porosity or permeability. Air cannot circulate in such heavy soils, and water is held in them by capillary action. Consequently evaporation rates from silt and clay soils are low, and such soils are not suitable for use in septic tank evaporation fields.